In the recovery of hydrocarbon values from subterranean formations, it is common practice, particularly in formations of low permeability, to fracture the hydrocarbon-bearing formation to provide flow channels to facilitate production of the hydrocarbons to the wellbore. In such fracturing operations, a fracturing fluid is hydraulically injected down a well penetrating the subterranean formation and is forced against the formation by pressure. By this procedure, the formation is forced to crack or fracture, and a proppant is placed in the fracture. The fracture provides improved flow of the recoverable fluid, i.e., oil, gas, or water, into the well. While a wide variety of fracturing fluids have been used, fracturing fluids customarily comprise a thickened or gelled aqueous solution which has suspended therein "proppant" particles which are substantially insoluble in the fluids of the formation. Proppant particles carried by the fracturing fluid remain in the fracture created, thus propping open the fracture when the fracturing pressure is released and the well is put in production. Suitable proppant materials include, but are not limited to, sand, walnut shells, sintered bauxite, or similar materials. As will be understood by those skilled in the art, the "propped" fracture provides a larger flow channel to the well bore through which an increased quantity of hydrocarbons can flow, thereby increasing the production rate of a well.
A problem common to many hydraulic fracturing operations is the loss of fracturing fluid into the porous matrix of the formation, particularly in formations of high permeability, e.g., formations having a permeability of greater than 2 md. Fracturing fluid loss is objectionable, not only because of cost consider-ations, but especially because it limits the fracture geometry which can be created in high permeability formations. In general, fracturing fluid loss depends on the properties of the rock in the formation, the properties of the fracturing fluid, the shear rate in the fracture, and the pressure difference between the fluid injected and the pore pressure of the rock matrix. In this regard, the properties of the fracturing fluid are those exhibited by the fluid in the formation as influenced, inter alia, by the temperature and shear history to which the fluid has been subjected in its travel down the well bore and through the fracture.
Thorough analysis of the problem of fracturing fluid loss in high permeability formations reveals that it is necessary to reduce "spurt". As used herein, the term "spurt" refers generally to the volume of fluid lost during fracturing because of early leak off of fracturing fluid before pores of the formation can be plugged, and/or an external filtercake on the surface formed. In the past, a variety of additives to the fluid have been employed, most being selected or designed to generate an external low-permeability filtercake quickly, under little or no shear stress (usually referred to as static conditions) in order to cover the pores and stop spurt. This approach is unsatisfactory since high shear stresses eliminate or severely limit the formation of external filtercake.
In general, the higher the permeability of a rock, the greater the spurt is likely to be. However, it has been determined that during hydraulic fracturing, spurt occurs principally at or near the advancing tip of the fracture, where new rock surface is being generated. The shear stresses that the fracturing fluid exerts on the surface of the rock are greater proximate the tip of the fracture because of the narrower fracture gap in that location. As indicated, the high shear stresses prevent the formation of external filtercakes of polymer and/or fluid loss additives by eroding the surface of the cake in contact with the fracturing fluid. Accordingly, to be effective, a fluid loss additive must be able to stop spurt under high shear rates.
Williamson et al (U.S. Pat. No. 4,997,581) describe the prior art utilization of a variety of inorganic solids, natural starches, and combinations of finely divided inorganic solids with natural starches. All these compositions are deemed by these patentees to be deficient for controlling fracturing fluid loss in moderate to high permeability formations. While these patentees attempt to provide an effective additive by the use of blends of natural starches and modified starches, their blends have limited application. For example, for formations having high permeability and high temperatures, e.g., 300.degree. F., natural and modified starches may not effectively plug the pores in the fracture walls. Finally, additives suggested by other workers in the art, while providing some fluid loss control, often are excessive in cost.
Accordingly, there has existed a need for a low cost additive or fracturing fluid which provides fracturing fluid loss control, and a method of fracturing a subterranean formation characterized by reduced fluid loss, under a variety of conditions which include both high permeability and high temperature. The invention answers this need.